Question

Imagine this C++ code (which uses clang’s blocks language extension).

auto now = std::chrono::system_clock::now();

std::async(^ {
  auto time = now;
  // ...
});

What will be the type of time? Will it be decltype(now) or decltype(now)&? If it’s the latter, how can I make sure it’s copied before the block runs?

Answer 1

The object is copied, and its type is decltype(now), as per the block language specification.

Stack-local objects are copied into a Block via a copy const constructor. If no such constructor exists, it is considered an error to reference such objects from within the Block compound statements.